$ A = \left[\begin{array}{rrr}2 & 0 & 1 \\ 0 & 1 & 1 \\ 1 & 1 & 1\end{array}\right]$ What is $ A^{-1}$ ?
Solution: $ = \left[\begin{array}{rrr}0 & -1 & 1 \\ -1 & -1 & 2 \\ 1 & 2 & -2\end{array}\right]$